Incipient sodium boiling superheats in bundle geometry under loss-of-flow conditions

Transient sodium boiling experiments were conducted using electrically heated pins arranged in 7-, 19- and 37-pin bundles to reproduce loss-of-flow conditions. The average degrees of superheat attained before inception of sodium boiling amounted to 47, 45 and 160C respectively with the 7-, 19- and 37-pin bundles. The experimental results indicated that the degree of superheat decreased with increasing number of heating pins in the bundle. This tendency can be explained from the fact that a larger bundle has a radially wider region of sodium saturation temperature at boiling inception comparatively - with the smaller cooling effect exerted by the peripheral subchannels - and consequently a larger number of active nuclei to trigger boiling and terminate the superheating phenomenon. No meaningful correlation was discerned between the degree of superheat and other factors like sodium velocity, rate of sodium temperature rise and intensity of applied heatflux. (author)